gnuplot only differences

There are a lot of gnuplot differences and it might be nice to fix those, but someone would have to take an interest. Octave is moving towards OpenGL for all plotting so most developers focuses on
bug fixes for that toolkit.

Nevertheless here are some visual gnuplot ONLY problems which need some attention:

won't fix or Octave only feature

fltk/ML axis_09, axis_10, axis_11, axis_12, axis_13: No, [x,y]axislocation' 'zero' (http://savannah.gnu.org/bugs/?40588). This is an Octave (and gnuplot) only feature. We can either blacklist, or visually skip over the comparison since it is blank for Matlab. Perhaps should be removed from demos or blacklist axis_09 in dump.m script

matlab comet3_01, comet.png: Only the beginning of the animation is printed. Matlab implements comet in a completely different way which means printing isn't really possible for them.